The goal of this project is to make simultaneous measurements of the microvascular flow by optical means and afford an independent measurement of material transport across the vascular wall. A system for collimated beta particle detection at a microscopic scale will be coupled directly with a reflecting microscope objective lens permitting continuous observation of the field. The feasibility of practicality of the system (called the "nuclear biology microscope") for use with intravital preparations has been demonstrated in experiments measuring the passage of 131I across the microvessels in cat mesentery. The signal input to the detector has been characterized and the spatial resolution and absolute counting efficiency of the system will be determined. A new silicon avalanche diode (Texas Instruments TIXL56) will be used at low voltage as a solid state analog of a gas ionization chamber. A multichannel analyzer system, which also permits data display, information retrieval and calibration, will be used to characterize the signal input to the detector. The complete system of detector and analyzer can selectively quantitate collimated beta counts in a "sea" of uncollimated gamma rays. Measurement of spatial resolution for beta particles will be achieved experimentally by using point source (30 micron) of radioactive material and improved by use of small-bore collimators, limited in bore size only by the obtainable count rate in the biological preparations. Transport in cat mesentery will be measured at arteriolar, metarteriolar anc capillary sites with 131I or 68Ga. Flow dynamics will be correlated with the transport studies.